The cartoon rendering, as you can see in the wireframe shots, use a fatter and
black object precalculated at the load time and drawn with inverse backface
culling.

This fat object contains only vertex buffer with the FVF XYZ ( without normals,
and so on), so it is only 30 % more memory usage than using only the normal
mesh.

The fat object is precalculated using the normal of each vertex, instead of an
uniform scale wich does not give correct result for angles.
The rendering is done using the ZBias function as mentionned before, to avoid Z
Fighting, and can involve Anti Aliasing for nicer borders.

The shading is a two layers shader, with the texture and an environment mapped
texture on it,

If you want to participate to the engine, just tell me, I like to work with
other programmer !